Is the に in 以上に mandatory in the following context?

Question

I came across the following sentence in a text about 俳句　(emphasis, translation and words in square brackets mine):

携帯電話で俳句を送り合って遊ぶ若者もいますし、外国人が日本人以上にすばらしい俳句を表現することもあります。　There are youngsters who play sending haiku to each other's mobile phones, and there's foreigners who can even make up more splendid haiku than Japanese [themselves].

I know that 以上 means "more", "above" and/or "beyond", but as far as I know, 以上 alone functions as an adverb, so I wonder what role is に playing here. Is it just marking 以上 as an adverb (i.e. signaling that 以上 modifies 素晴らしい俳句を表現する), or there is something else going on? Would the sentence be okay if we remove this に altogether?

I opened a related question dealing with the meaning of 以上に itself, please check it out too.

Answer 1

In short, you cannot remove に from 以上に here.

The sentence below would be ungrammatical:

*外国人が日本人以上すばらしい俳句を表現することもあります

The sentence below would be fine and would mean more or less the same as the 以上に version:

外国人が日本人以上のすばらしい俳句を表現することもあります

(The structure is different, though - in this case 日本人以上の and すばらしい both modify 俳句 in parallel.)

"以上 alone functions as an adverb" - I think this can be true, but I feel like it should be treated as a separate sense, typically applied to amounts, frequencies, etc. Example:

私は同じことを二度以上言わない

With に it would be awkward if not ungrammatical:

?私は同じことを二度以上に言わない

(There might be something more to be said about the last point, but this is all I can say off the top of my head.)